For recreational, norml lists this info :

Transfer of one ounce or less for no remuneration by persons 21 years of age or older is no penalty.
See:
• Amendment 64: The Regulate Marijuana Like Alcohol Act of 2012 | Yes on Amendment 64: The Colorado Campaign to Regulate Marijuana Like Alcohol

For card holders, the only thing I'm sure of is, you can not take medical stuff you got at a store and give it to someone else. Giving someone bud you grew yourself would be legal only if you receive nothing in return. Not sure how this applies to regestered caregivers and their patients, I assume remuneration is legal in that case.

As far as Craigslist and "donations" it is very legally "grey" at this time but defiantly outside the laws set in place for recreational sales.

Best bet, if you need medicine, get a rec. If you must get from craigslist, be very cautious, you should be more worried about getting hurt or robbed or worse. Legally the the person providing the cannabis and receiving the donation is more actionable than the person making the donation.

And if your just looking to sell your stuff, stop it, there is enough bad/overpriced stuff on craigslist as it is, if your that good of a grower, get hooked up with a dispensary.